Letter Position Dyslexia

Many word-reading models assume that the early stages of reading involve a separate process of letter position encoding. However, neuropsychological evidence for the existence and selectivity of this function has been rather indirect, coming mainly from position preservation in migrations between words in attentional dyslexia, and from nonselective reading deficits. No pure demonstration of selective impairment of letter position function has yet been made. In this paper two Hebrew-speaking acquired dyslexic patients with occipito-parietal lesions are presented who suffer from a highly selective deficit to letter position encoding. As a result of this deficit, they predominantly make errors of letter migration within words (such as reading "broad" for "board") in a wide variety of tasks: oral reading, lexical decision, same-different decision, and letter location. The deficit is specific to orthographic material, and is manifested mainly in medial letter positions. The implications of the findings to models of reading and attention are discussed.